Separation by crystallization



Jan. 19, 1960 n. H. WHITE SEPARATION BY CRYSTALLIZATION 2 Sheets-Sheet 1Filed Dec. 20, 1954 INVEN TOR. DONALD H. WHITE A rromvsy United StatesPatent 2,921,682 SEPARATION BY CRYSTALLIZATION Donald H. White,Phillips, Tex., assignor toPhillips Petroleum Company, acorporation ofDelaware Application December 20, 1954, Serial No. 476,533

6Claims. (Cl. 21081) This invention relates to separation bycrystallization. In one of its more specific aspects, the inventionrelates to an improved apparatus for the separation of com.

ponents of mixtures by fractional crystallization. In an other of itsmore specific aspects, the invention relates to an improved method andmeans for separating crystalline and non-crystalline material in avacuum filter.

Separations of compounds may be effected by distilla tion, solventextraction and crystallization. Although distillation and extraction aregenerally preferred because ofeconomy and convenience of operation,there are some instances in which such processes-cannot be successfullyutilized. Many chemical isomers have similar boiling points andsolubilities and'cannot be separated satisfactorily by distillation orextraction. Separation by means of fractional crystallization can besatisfactorily utilized in many cases in making such separations.Fractional crystallization has one great advantage over other methods ofseparation in that it is the only separation method which theoreticallyoffers a pure product in a single stage of operation in systems in whichthe desired component of a mixture solidifies at temperatures above thatat which the other components solidify. Thus, whereas distillation andextraction theoretically require infinite stages for a pure'product,crystillzation inmany cases requires only one. This is because of thenature of the phase equilibria is distillation and extraction, while bycrystallization, sub.-

stantially pure crystals can be formed from many solu-' tions in onestage,'although the desired component may be of low concentration in theliquidifeed.

Crystallization is thus well suited, not only to the separation of manychemical isomers which can be separated V by no other means, but also tothe purification of many compounds which cannot be economically purifiedby other means. Whereas one stage of crystallization theoreticallyofiers a pure product, attainment of this ideal stage has beendiflicult. Complete removal of occluded impurities without substantialloss in yield is required. This invention results in a very closeapproach to the ideal cry s tallization stage.

Methods of separating a pure component from a mixture have been devisedwhereby the mixture to be separated is introduced into a heat exchangezone wherein a mixture of crystals and liquid is formed and that mix Onesuch means for purifying crystals mechanism whereby this displacingliquid 'corresponding 'to the melt' improves the purity if the finalproduct is not completely understood. However, it is' presently believedthat the substantially pure material which is'refiuxed through at leasta portion of the crystal mass displaces occluded impurities in theinterstices.

the crystals.

ing toward the melting zone. crystal mass which approaches the meltingzone does not contain any appreciable amount of impurities and theresulting product which is removed from the melting zone is of extremelyhigh purity.

In order to separate constituents of mixtures by tractionalcrystallization, it is necessary to adjust the tempera" ture of themixture to one which is below the temperature at which crystals form ofany one 'ofthe pure constltuents. For example, when para-xylene isseparated from a mixture of isomeric C ralkyl benzenes, it is necessaryto cool the mixture to a temperature in the neighborhood of about 57 toabout 78 C. Generally, the desired'constituent is relatively dilute inthe liquid feed substanmixtures and, upon such cooling, crystals and-atial'volume of mother liquor are-formed.

An effective system for removing mother liquor from-. the crystals is avacuum filter, such as a rotaryor belttyp'e' vacuum filter. In such asystem, the crystal slurry. is deposited on the filter medium and motherliquor is pulled through the crystals by a vacuum pump. Wherethe systemoperates at lowtemperature, such as that de-' scribed above, one of theproblems is to maintain a high thermal efficiency in the filter system.

In handling very cold crystal slurries, it has been'found desirable toemploy a closed filter system, so that gaseswhich are pulled through thecake of crystals in the filtermedium by the vacuum pump, are recycled tothe filter medium. A pressure at least equal to atmospheric pressure,preferably a slight positive pressure, of these re-* cycled gases ismaintained about the filter medium sothat fresh air does not enter thesystem to thereby lower its.

thermal efficiency. Also, fresh air introduces water vapor which is veryundesirable when handling cold slurries of crystals, since the watervapor freezes on the filter medium and causes the filter medium tobecome plugged.

In the vacuum filter systems, such as have been described above, a dryvacuum pump has been used in the. prior art. The dry vacuum pumpoperates on the en-- trained gas which has been separated from themother:

liquor removed from the'filter cake. However, such a dry vacuum pumpdevelops heat in.-its operation and coolers have beenrequired to lowerthe temperature of f the gases discharged from the pump before beingreturned to the filter. Thus, a need exists for a vacuum filter sys-;

tern having an improved thermal efliciency for handling crystal slurriesat low temperatures.

An object of this invention is, therefore, to provide an pure componentimproved system for the separation of a from liquid mixtures.

Another object is to provide'an improved vacuum,

filter system for removing mother liquor from a slurry of crystals.

A further object is to povide a vacuum filter, system for;v handlingvery cold crystal slurries which has an improved thermal efiiciency anddoes not become plugged. Other objects of the invention will be apparentto those skilled in the art upon study of the accompanying closure.

Patented Jan. 19, 196i) At least, a portion of the pure material isrefrozen on the surface of V A high yield of product is obtained sincethe high melting product refreezes from the reflux stream as it comes incontact with the cold crystal mass mov- Thus, the portion of the' In.accordance vla'th thisinvention, a wet vaeupm pump n is used in a vacuumfilter system for removing mother liquor from a slurry of crystals. Thewet vacuum pump handlesgtliei total mothe iqu str am; em ed omv i the'filter, including-entrainedgases; and since; the; entrained gases aretherebycontinually. in contact withthe cold motherliquor-until justbefore they :are returned to the filteringimedium itself, the entrainedgases'do' not .beconflyheated inithe vacuu n pump The result is thatcoolers forthe entrained gases found in vacuum filter sys 'ernspi theprior art can be eliminated-with consequent savings in] equipment andinvestment. In accordance With;.thisinvention the entrained gases aremaintained at.

sub stant iallyl the .same temperatureas the mother liquor tlrroughoutfthevacuum filtersystem and this provides: an improvement in thethermal efficiency of-the system; In thosegfilter; systems which areoperated at. very-lowtemperat re; littler; o; de se h r h. r,"

7 for. completely removing. any crystalline material" which remains onthefilter medium to, plug ofi;the filter me dium. I .Theover-allimproved yvacuumi, filter system; of this invention is particularly welladaptedtq handle cold crystal :slu'r'rieS, since ice formation isminimized and means "are provided: for removing, ice ;or othercrystalline 7 material from the filter mediumvery effectively, tetherebyprevent the. fi ter medium :from becoming-plugged. It will-the readilyappreciatedbyv those skilled in the art that" my invention has broadapplication to vacuum filter systems :Whenever iitf isydesirable' thatentrained gases.

in the motherliquor .be'maintained atsubstantially the v sametemperature as the mother liquor itself." A preferred embodiment 'of :myinvention resides in ;an apparatus wherein the mother liquor is removedfrom the crystals in a rotary vacuum filter while introducing thecrystalsfinto anruprightzpurification chamber.. The crysdownstreamendvthereof. At least a, portion of the V crysc ountercurre'ntly throughatleast a,' portion of the crystal "masslsoias. todispla ne occludedimpurities therefrom;

Althoug'h, asl have pointed out .hereinbefore, this invention isparticularly'applicable to systems jin which the temperature atlwhiclicrystals of the desired 'pure componentform is. considerably higherthanthe temperature to whichithe component. is .cooled in the liquidmixture .t'als are .moved downwardly in the purification chamber asanielongated crystal .mass to almelting; zone in the;

, tals is melted 'andasmall portion of the melt jisjdispla'ced toform-crystalslthereof, thisprocedure canibe advantageously. utilizedinpractically any system J to which fractiofialcrystallizationfisapplicable sons to increase the eificiency of theprocess. This: invention is applicable Jtothe separations in many.multicomponent'systems," the cornponentsoffwhich have practically'thesame boiling point and are; therefore; diflicult to separatebyfractional distillation, one mixtures which have diverseboiling points"but which form azeotropesor .are heat sensitive.

The effective separation of components of such mixtures p thepurification of a component of, .say 15 to 25 percent purity,j so a sItoefiect-a product purity upwards of 98.

p ereent lnaord'er to fillustrate 'some of the systems. to

which xtheinvention is applicahle jthe following cornpounds are groupedwith respect to their boiling points.

Group A B.P., 0. RR, C.

Benwno 5. 5 n-Hoxane 69 94 n-Hept e 98. 52 90. 5 Carbon tetraehlorrd 7122. 8 Acrylonitrile. 79 82 Ethyl alcohol 78. 5 117. 3 2,2-Dimethylpent79' -125 3,3-Dimethylpenta 86 c lvlethyl ethyl keton 79. 6 -86. 4 Methylpropionatm 79. 9. ;87. 5 Methyl aerylate. '80. 5 1,3.C nlnhpxarliene V V'80. 5 -98 ZA-DimethyIpentane 80. 8 123. 4 2,2,3-Trimethylbutane -80. 925 Oyclohexane. 81. 4 6. 5 Acetonitrile 82 Cyclohexene 83 r -103. 72-Methylhevane 90 119 3-Methylhmane 89. 4 1l9. 4

Group B B.P,, 0 F1? "0.

Methyl oyclohexane 100. 3 126.3 Qyclohexane- 81. 4 V f 6. 5: n-Heptane Lp '98. 52- 90. 5; 2,2,4Trimethylpentane .(isooctane) 99. 8 W. 4 Nitrnmethane 101 29 p-D 101;.3 11-.7 2.]?pntannne l 101, 7 -77. 32-Methyl-2-butanol 101. 8 p 11.9 2,3-Dimethylpentane. 89. 43-Ethylpentane 93. 3 v :94. 5

Group C B.P., 0 RP C.

Toluene V 110. S 'Methylcyelohexane 100. 3 +126. 32,2,3,3-'Ietramethylbutane" .106. 8 .104 2,-5-Dimethy1hexane- 108. 25-91 Aniline 4 'lnlm no Bon'ven'e G roup 7 BER} O F-.P.,- O;

Oarbon'tet'rachloride 7 77 22. 8 Ghlor -61 i 63. 5 CS: 46.3 -'108.6Ar-atone 56.5 95

Group-F i B.P., C 1 .1%, .0.

ortho-X e 7 14-1 .2.'. 1 InetarXylene V 138. 8 47. 4 para-Xylene Y 138.5- 13.2

Group G B.' P. 0. RP; o.

. 0rtho-Cymene; e 7 175.0 ra. 5 metaCyrnene" 175. 7 -25 pare-Qy'mene.176. 0 73. it Group Hi i 3.1 e 01' Mr, 0

bimethyl phthalate 282 l 5.5 Dimethyl isophthalatenni 124 67 V j. i V(12mm) Dimethyl terephthalate 2c '14). 6

Group I an, O o in .P., C.

ortho-Nitrotoluene; 222. 1 2-? V metafljlitrotoluenen 231 15 5paraFNitrotolueneqnfl- 238 h 5L3 System. consistinglof' any;combinationjof two o t e o p n within any one i l eet nsjmax-beseparated by the process of the invention," as well as systems made upof components selected from difierent groups; forexample, benzene may beseparated from a benzene, n-hexane or n-heptane system in which thebenzene is present in an amount greater than the eutectic concentration.In the same manner, para-xylene may be readily separated from a mixtureof paraand metaxylenes or from pma-, meta-, and ortho-xylenes, or from amixture of para-, meta-, ando'rtho-xylenes plus ethylbenzene and othercompounds. Benzene may also be separated from a mixture of tolueneand/or aniline. Multi-component systems which may be effectivelyseparated so as to recover one or more of the components insubstantially pure form" include -2,2-dimethy1pentane,ZA-dimethylpentane, 2,2,3 trimcthylbutane, methylcyclohexane,2,2,4-trimethylpentane, and carbon tetrachloride, chloroform, andacetone. The invention is also applicable to the separation ofindividual components from a system of cymenes and 'a'system includingthe xylenes.. This invention canalsobeutilized to purify naphtha-J lene,hydroquinone, (1,4-ben'zenediol), paracresol, 'para dichlorobenzene, andsuch materials J as high melting waxes, fatty acids, and high molecularweight normal paraflins. The system can'also be used toseparateanthracene; 'phenanthrene'fand carbazole, Furthermore, the'system can beused to separate. durene (l,2,4,5-tetramethylbenzene).from Cm'aromatics.In cases where the,

material to be purified'has. a-relatively high crystallization point,thesimpure material is-raised toga temperature at Zwhich only aportionofthef constitutents. are in a crystalline state'and the resultingslurry is handled at suchfl One special class 7 process. The process isalso applicable to theconcentra-f- Figure 4 is an enlarged detaileddrawing of stationary;

plate 81 and slide valve-79; and

Figure 5 is an end ,view of the rotary Figure 3.

Referring now to Figure l,'a mixture of materials from which at leastone constitutent is to be separated, is fed, through a conduit 1,through a control valve iiland into a scraped surfacechiller 5; Thetemperature'of thefeed mixture in ,chillerj is adjusted so as to obtaincrystals oi at least aportion of at'least oneoi the constituents of e.ur chiller which is suppliediwith refrigeratiommeans that are adequateto adjust the temperature of the mixture to that necessary to obtaincrystals of at least a portion of a least 1 oflhe qnst nt saw Aswi bepparent from the discussion hereinabove the temperature to;which.themixture isadjusted in game; 5 depends en-l r y. p n th pe i teed; mi ursi e; v r us materials in the feed have different solidification pointsof the accompanying drawings,

c sectional view of awet filter shown in i Chiller 5,can,b e anyconventional type,

and since the solidification point of any given constituent of the feedmixture is dependent upon the composition of the feed mixture. isomericC alkyl benzenes is fed through conduit 1 to chiller 5, the temperatureof that mixture is lowered to 'a' temperature in the neighborhood of 57-to- 78 C. When the desired component of the feed 'mixture is relativelydilute in the feed mixture, the temperature at which crystals will formwill be relatively low. If the desired constitutent is relativelyconcentrated in the feed, the temperature at which crystals will form.

will be relaively high. A cooling medium is passed through a jacket inthe wall of chiller 5 via a conduit 7 and a conduit 9 to accomplish theaforedescribed adjustment of the temperature of the feed mixture.

The resulting slurry of crystals is passed from the chiller 5 into afiltervll wherein the noncrystalline material is separated from thecrystals. Filter 11 is a vacuum'filter and, for illustrative, purposes,a rotary vacuum filter is shown'in Figure 1.' Other vacuum filters, suchas abelt-typevacuum filter, can also be used. A constant level 13 of thecrystal slurry is maintained in the bottom of filter 11 by a=levelsensing device 15 which in turn controls the rate at .which thefeed mixture is introthrough the slurry of crystals in the bottom offilter 11, a cake of crystals and mother liquor is formed on the sur-;face of drum 17 and mother liquor is removed from this cake through line21 by a wet vacuum pump 19 and is" passed to a liquid-gas separator 23.The liquid mother liquor is removed from separator 23 viaa. conduit 25and entrained gases are removed from separator 23 via a conduit 27 andare returned via a conduit 29 to filter 11. As was stated above, avacuum is also applied to the upper fportion offdru rn17 by a wetvacuumpump 31 v through aiconduit 33 which connects pump 31 to the upperportionof 17; Similarly, the combined stream of mother liquor andentrained gases is discharged from pump 31 :via a conduit 35 ;0 aseparator 37. Mother liquoris removed from separator 37 via acon- V dpft39jandis combined with mother liquor removed fror'n separator 23 viaconduit-25. The entrainedgases from the mother liquor in conduit 35 areremoved therefrom in separator 37 passed via a conduit 41to combine withthe entrained gases from separator 23 in, C Ol]dl 1lt 29 to bereturnedto filter 11. A single wet vacuum pump may be used, in accordance withthis invention, in place of the twowet vacuum pumps 19 and 31.: thisevent, itis usually desirable to provide suitable control means, notshown, so thata differentialreduced"pressure can be maintained in theupperjand lower sections of drum 17.

iReferring' now to Figure 2 a diagrammatic sectional .viewofthe wetvacuum pumps '19 and 31 is shown. Such pumpsare well known in'the artand may be described as a centrifugal displacement type of pump havingan elllptlcal casing 43 partlyfilledwith liquid 45, in which a round,multi-blade rotor-47 revolves freely. Rotor 47 comprises a pl'urality ofcurved rotor blades 49 which" pro ect radially from the hub 51' andform, with thejside shrouds, a series of buckets. Rotor 47 revolves in aclockwise direction at a speed high enough to throw the liquid out'fr omthe center by centrifugal force, solthlatf there results a solid ring ofliquid revolving in the casin g" 43 a t the same speed as'the rotor 47,but hugging the walls of the .elliptical'casing 43. This action causesthe liquid'to ent'erand recede from the buckets in the rotor" When, forexample, a mixture of" deliver, the compressed gas to the outlet port.The drawing-in of gas, and discharge an instant later, takes place.

twice in one revolution. Such a wet vacuum pump is manufactured byNash'Engineering Company, Norwalk, Connecticut, and is called aNash-Hytor' pump or compre'ssor.

.Referr'ingnow to Figure 3, a diagrammatic sectional view of filter 11is'shown. Drum 17-comprises a filter medium or screen'61 upon which thecake of crystals and mother liquor is formed. Screen 61 can be formedfrom any suitable filtering means, such as cloth, paper, felt, glassfabric, synthetic fabrics, perforated or porous (sintered) metal or anycombination of these orother filtering media as is necessary toeffectively retain crystal! line. solids and pass liquid in thefiltration of crystal slurries. Screen 61 is preferably a 24 x 150 meshDutch j Weave'Mon'el filter screen on topv of a suitable support,

Drum 17 is totally enclosed in a heavily 7 not shown. insulated housing63.' The upper portion of'housing 63 isfit'ted with windows 65, 67 inthe sideiand with-windows 69, 71 on either end of housing 63.' Drum 17'isf rotated by a rotatable shaft 73 and is divided into 16 sections 75by radially extending members 77, as'shown. Each of the sections 75' isconnected to an opening in a slide valve 79 By proper'construction' of astationary plate to apply pressure or vacuum to any of sections 75 forrotates; The slide valve 79 comprises a top section 83 and a bottomsection 85, as is shown in Figure t.

- additional section may be provided if ineansifor supply} ing blow-backto the drum are desired;

after.

V Further in accordancewitlrthisinventiomaspray pipe:

18 1 of slide valve 79, as shown in Figure 4, it is possible any desiredportion of the cycle through which the'jdrum I heatingfelement 127 wherea po'rtio'n of the'crystals isa duit 89 to a pur fication means tobedescnbed here n-' 8,6 isprovided for discharging a confined, h otfluid stream thereth'rough jagainst the filter cake to completelyremove:

crystals" from the surface ofjscreen 61. Ahotfluid under 1 pressure isconveyed througha conduit 88 to spray pipe 86 to be. discharged against'screen 61, and therebyto remove any crystalline material from ,screen61 not rev moved by scraper 87. .Spray pipe :86 is positioned adjacentto screen 61 immediately below scraper '87. Thus the heated fluid issprayed against the filte r' medium rriediately downstream of thescraper with respect to-the direction ofmovement of the filter mediumpasuthe.

scraper, Any condensible fluid can be used for this PHI".

pose whichis inert with respect to the. mixture being filtered and has afreezing point sufficiently low that the fluid does not freeze uponstriking the jfilter medium. A preferred fluid is the motor liquoritself which is heated by means not shown to thevapor state. Obviou'sly,the use of heated mother liquor possesses the advantage of notintroducing a foreign material into the 1 system. For best "results, themother liquor is heated until-its vapor'is superheated. Thus, thesensible heat of the fluid aswell as the more important latent heat 'ofI vaporizationare taken dvantage' of by using a con-1 'densible fluidfor this purpose; i

' Referringnow to Figure 5; an end .view of filter l l back to chiller 5wherein it is utilized to enrich the feed ofztheafilter: cleaning;means, spray; pipe 86, and its relato, such an"exte'nt that the forwardface of piston coincides with the. side of purification chamber 93.Piston '95 :hasa porous face 103 which permitsv liquid in the crystals;in chamber 91 to passther'ethrough and out of chamber v91 viaconduit105l It is desirable to supply crystals to purification chamber93 at a temperature which is sufiiciently highv to prevent freezing ofthe crystalmass to 'an impervious plug. 'To' this end, heating materialis passed through heating jacket 107 about chamber, 9 1' by means of aconduit 109 and a conduit 111. Duringthis least a portion of the thusproduced melt is removed vi conduit as a result of thecompacting.

' The crystals which have been moved into chamber 93 are moved as anelongated "crystal mass dow'nwardly .by a piston 113. Piston 113 iswithdrawn by means of'fluid flow through a conduit 115 and actingagainst a piston 117, which is connected to piston 113 by a piston rod119. Piston 113 is moved in a downstream direction by flow of fluidthrough a conduit 121 and against piston 117. Piston113 also comprises aporous face 123 which permits the passage therethrough of liquid fromthe crystal massas it is compacted and out of chamber. 93 via conduit125. '7

In this manner, the crystals are moved downwardly through chamber 93 tonmelting zone in the lower end thereof to come into heat exchangerelationship with a melted. Heating element 127 maybe any type 'ofheating*device,"such"as coilsthroiigh which a heat exchange fluid ispassed, or may be an electrical heating element provided'within orwithout ;chamber'93. A portion of the melt thus produced is displacedcountercurrently to the movement of the crystals and through a .portionof the crystal mass so as to displace impurities from the crystal mass.The impurities'which are displaced from thecrystalmalss arere'movedthrough a filter 129 and a conduit 131. Under some conditions ofoperation, the concentration of material correspondingto the purifiedmaterial in the stream removed. through conduit 131 is higher than itsconcentration inthe feed mixture introduced via conduit 1 to chiller5,1I n"t bisevent, the stream in conduit 131 can be passedby ,a conduit,not shown,

mixture; This results in formation of larger crystals. than .canbeformed from ,a feed mixture containinga lowerconcentration of'thedesired component. If the concentration of the'desired component in thestream removed via conduit 131 is relatively low, that stream may bedisposed of asis'desiredq Purified product is removed from chamber93through a conduitf133...

. In order to betterv illustrate my invention, I. have pro: 7 vide'dvthe following example which intendedtoexemplify the invention but notundulyto restrict it. I

3 Exa j' 1 A e d mi mm'c mrris as fi fs n fb s sh t ara en resent b miht mac-mes. 33- Prcflt by weight meta-xylene, 27.5 percent. ,by weightethyl azsn aud ts. re ent b W ight olu e urp i d 9 thesystem at the-rateof 1000- gallons per hour-through conduit 1. "Liquid material recoveredas displaced liquid from chamber 93 through filter 129 Comprises 53.1percentpara-xylene, 9.8 percent ortho-xylene, 18.7 percent meta-xylene,15.6 percent ethyl benzene and 2.8 percent toluene. This stream issupplied to chiller at 165 gallons per hour, together with the feedstream described hereinabove, the resulting composite feed comprising22.1 percent para-xylene, 16.3 percent ortho-xylene, 31 percentmeta-xylene, 26.2 percent ethyl benzene and 4.4 percent toluene at 1165gallons per hour. That material is cooled to temperature of 76 C. inchiller 5 with the resultant formation of 15.1 percent solids. Theslurry of mother liquor and crystals is passed to filter 11 whereinmother liquor having a para-xylene content of 6.75 percent is removed ata rate of 890 gallons per hour through conduits 21 and 33. In accordancewith this invention, a wet vacuum pump is used to remove mother liquorfrom filter 11, by appling a vacuum equivalent to 20 inches of mercuryto filter 11, and to discharge entrained gases at substantially the sametemperature as the temperature of the mother liquor and at a positivepressure equal to 0.5 inch of water. The crystal material from which themother liquor has been removed is 70 percent solids and is at atemperature of 76 F. That material is supplied to compacting chamber 91wherein the crystal mass is further compressed while being heated to atemperature of 23 C. and a stream of 40 percent solids at thattemperature is moved to purification column 93 at a rate of 275 gallonsper hour. The crystals are compacted in column 93 and are moved as acompact mass downstream to the heating zone wherein the crystals aremelted and a portion of the melt is displaced upstream through thecompact crystal mass. A product which is about 99 percent para-xylene isremoved through conduit 133 at a rate of 110 gallons per hour.

It will be apparent to those skilled in the art that variousmodifications of this invention can be made upon study of theaccompanying disclosure. Such modifications are believed to be clearlywithin the spirit and the scope of this invention.

I claim:

1. In a vacuum filtering process for separating crystalline andnon-crystalline material, a method for preventing obstruction of afilter medium used in said filtering process comprising passing a slurryof said crystalline and noncrystalline material in contact with saidfilter medium to form a crystalline cake thereon; scraping saidcrystalline cake from said filter medium; and directing under pressure astream of superheated vapor of the non-crystalline material against saidfilter medium immediately following said scraping step, thererby toremove from said filter medium crystalline material adhering thereto.

2. In a vacuum filtering process for separating crystalline andnon-crystalline material, a method for preventing obstruction of afilter medium used in said filtering process comprising passinga slurryof said crystalline and noncrystalline material in contact with saidfilter medium to form a crystalline cake thereon; displacing saidnon-crystalline material from said cake through said filter medium withgas having a temperature such that said crystalline material does notmelt and recrystallize within the passages of said filter medium,thereby forming a stream of gas entrained in said non-crystallinematerial; compressing said stream so that said gas is maintained atsubstantially the same temperature as said non-crystalline material dueto the contact therebetween; separating said gas from saidnon-crystalline material; recirculating said gas to the filtering zonefor reuse in displacing non-crystalline material from cake on saidfilter medium; scraping said crystalline cake from said filter medium;and directing a stream of superheated vapor under pressure against saidfilter medium immediately following said scraping step, thereby 10 toremove fr'om said'filter adhering thereto. i

3. A process for fractional crystallization comprising adjustingthe'temperature of'a mixture of materials so as to obtain a slurry ofcrystals of a component of said mixtureinnopcrystalline materialthereoflpassing said slurry to a closed filtering zone; passing saidslurry in contact with a filter medium to form a crystalline cakethereon; displacing said non-crystalline material from said cake throughsaid filter medium with gas having a temperature such that saidcrystalline material does not melt and recrystallize within the passagesof said filter medium, thereby forming a stream of gas entrained in saidnon-crystalline material; compressing said stream so that said gas ismaintained at substantially the same temperature as said non-crystallinematerial due to the contact therebetween; separating said gas from saidnon-crystalline material; recirculating said gas to the filtering zonefor reuse in displacting non-crystalline material from cake on saidfilter medium; scraping. said crystalline cake from said filter medium;and directing a stream of superheated vapor under pressure against saidfilter medium immediately following said scraping step, thereby toremove from said filter medium all crystalline material adheringthereto.

4. In a vacuum filtering apparatus having a closed chamber with slurryinlet means 'and solids discharge means and a filter medium within saidchamber, an improved gas recirculating system comprising in combinationconduit means connected to said closed chamber downstream of said filtermedium; a liquid-seal gas pump adapted to pump gas-liquid mixtures andutilize the liquid in said mixture for the required liquid seal, saidpump being operatively connected to said conduit means to draw gas andliquid through said filter medium and from said chamber; a gas-liquidseparator downstream of said pump; means for removing liquid from saidseparator; and conduit means connecting said separator and said chamberfor recirculating said gas thereto.

5. A vacuum filtering apparatus comprising a closed chamber havingslurry inletmeans and solids discharge means; means for maintaining aconstant slurry level within said chamber; a continuous filter mediumwithin said chamber partially in contact with the slurry and rotatablearound an axis parallel to the surface of said slurry; a liquid-seal gaspump adapted to pump gas-liquid mixtures and utilize the liquid in saidmixture for the required liquid seal; conduit means connecting the inletof said pump with said chamber so that gas and liquid are moved throughsaid filter medium and from said chamber; a gas-liquid separatordownstream of said pump; means for removing'liquid from said separator;and conduit means connecting saicl separator and said chamber forrecirculating gas thereto.

' 6. An apparatus for separating crystalline from noncrystallinematerial comprising a closed chamber having a slurry inlet means andcrystal discharge means; a filter medium within said chamber; means fordepositing a slurry of crystalline and non-crystalline material uponsaid filter medium; scraper means for moving crystals from said filtermedium into said discharge means; distributing means for directing astream of pressurized vapor against said filter medium positionedposterior of said scraper means with respect to the direction ofmovement of said filter medium past said scraper means; conduit meansfor supplying superheated vapor to said distributing means; aliquid-seal gas pump adapted to pump gas-liquid mixtures and utilize theliquid in said mixture for the required liquid seal; conduit meansconnecting the inlet of said pump with said chamber so that gas andliquid are moved through said filter medium and from said chamber; agas-liquid separating means positioned medium all crystalline materialpyni txeam Q aid pump; mid m a sion sy l ns m ,667 46 from saidseparating means to said chambgar, 13 1,54 g g A I 2,081,296

Referpnces'C ited; the file; of patmtl 2,107,664 V UNITED STATES PATENTS5 7 68 8 1,576,137 Johnson ..Ma1'. 9 1926

